The present invention relates to the manufacture of foamed structures. In particular the invention relates to the use of fumed silica, including admixtures of fumed silica and normally liquid chlorofluorocarbons that are in the form of dry free-flowing powders, in the manufacture of such structures.
Foamed structures may be manufactured from many polymers. In some instances, for example the manufacture of polyurethane foams, the foaming of the polymer is accomplished essentially simultaneously with the polymerization of the polymer. The constituents of the foamed polymer, usually in the form of prepolymers and gaseous or liquid blowing agents, are admixed under conditions that facilitate both polymerization to form the polymer and foaming so that the result is a foamed structure. Such techniques are known in the art, for example, as described in Canadian Pat. Nos. 703,814 and 705,938, both in the name of C. B. Frost, which issued Feb. 16, 1965 and Mar. 16, 1965, respectively.
In other instances, for example the manufacture of foamed structures from thermoplastic polymers a solid, liquid, or gaseous blowing agent is admixed with solid polymer and extruded to produce a foamed structure or such blowing agents are injected into molten polymer prior to extrusion of the polymer.
Foamed structures are manufactured commercially for a wide variety of end uses, including the construction industry, e.g., as insulation and in wall structures, in the packaging industry, e.g., as containers, in the furniture industry and to make more economical use of polymers.
An important segment of the commercial foamed structure market is that in which the polymeric component is a polyurethane. Such structures are usually manufactured by admixing a polyol, e.g., polyhydroxy compound of the polyether type, and an isocyanate, e.g., toluene diisocyanate, 4,4-diphenylmethane diisocyanate or a polyisocyanate prepolymer. The blowing agent, especially in the form of a normally liquid chlorofluorocarbon, is usually added with the polyol. Other additives, especially water and a polymerization catalyst, may also be added with the polyol. On mixing the polyol and polyisocyanate, an exothermic reaction occurs. The heat liberated causes the normally liquid chlorofluorocarbon to vaporize with the result that a foamed polyurethane product is formed.
While polyols intended for use in the manufacture of polyurethanes are commercially available, polyols may also be available from other sources, for instance, by-products of the manufacture of other materials. Examples of such by-products are polyol by-products obtained from processes for the manufacture of polyesters. With some grades of polyols, frequently the less expensive grades, the foamed polyurethane obtained may be of poor or unacceptable quality for commercial use. In particular such poor or unacceptable quality foams may have a nonuniform distribution of the cells of the foam. Improvements in the manufacture of foamed polyurethane would be beneficial, especially with regard to the economics of the manufacture of foams of commercially acceptable quality.
It has now been found that the addition of small amounts of fumed silica to the constituents used to manufacture foamed polyurethane may improve the quality of the product obtained. It has further been found that in the manufacture of structures of foamed polymers, especially thermoplastic polymers, admixtures of fumed silica and normally liquid chlorofluorocarbons containing at least 4.5% by weight of silica may be used, the chlorofluorocarbon being the blowing agent used in the manufacture of the foam. Such admixtures are in the form of dry free-flowing powders, thereby permitting the addition of the blowing agent to the polymer in the form of a powder.